Unusual freezing problem shortly after starting

I’m having a strange problem with my N64. It works and starts up correctly and I can play games for maybe a minute or two, but then it always locks up and freezes. The screen freezes and it doesn’t respond to the controller. It does this every single time. However, an odd thing I noticed last night was that, when it froze during a certain game, the background music continued playing as normal. Has anyone experienced this before with their N64? Also, it’s worth mentioning that it has an off-brand expansion pack, which I’ve heard can sometimes cause malfunctions. Does anyone have any suggestions? I want to play my games!! :’( Any help is appreciated. Thanks!